In stationary (staring) imaging systems a scene is typically optically focused on a matrix detector which is composed of a plurality of detector elements arranged in a matrix. The scene is focused onto the plane of the matrix detector such that each of the detector elements is irradiated by a particular portion of the scene, and each detector element measures the quantity of the radiation it receives from that portion of the scene.
Different detector elements generally produce different measured values for the same light input. If uncorrected, this lack of uniformity among the detector elements gives rise to fixed pattern noise which can seriously affect system performance. In the infrared region, for example, this fixed pattern noise can readily mask the low contrast images common in that portion of the spectrum.
The standard solution to this problem is to calibrate each of the individual elements in the matrix detector. One prior art method of calibration is shown in FIG. 1. In this method, a known high value of radiation (H.sub.v) and a known low value of radiation (L.sub.v) are supplied to each detector element, and the element's measured values are plotted. A `calibration line` 15 is drawn through the two points on the graph, and this line is used to convert measured values to actual values during operation of the detector at values intermediate to H.sub.v and L.sub.v. In general each element will have a different calibration curve.
This prior art method requires a separate calibration procedure, however, and therefore it cannot be performed in real time (i.e. while the detector is imaging the scene). It is desirable to perform the calibration in real time because, due to drift, the calibration of the detector elements changes over time, and a `calibration line` calculated during a separate calibration procedure, may no longer be accurate during operation of the detector. Also, this method uses a straight line to approximate the detector element's behavior. In reality, however, the detector element's response is not linearly dependent on the energy flux and varies from element to element. This method, therefore, introduces inaccuracies into the calibration for values far from H.sub.v and L.sub.v.